OpenJDK 12 for CentOS-based alpha Docker images

The default OpenJDK is now in version 12 on CentOS, therefore the next Docker images based on CentOS will be using this JDK (instead of OpenJDK 1.8).

As a first step, this will be the case only on the alpha and alpha-light images which include Simplicité version 4.0.P24a (master branch).

Please report any issue you may experience with this up-to-date OpenJDK